Device for spectrographic analysis of a liquid metal

ABSTRACT

APPARATUS FOR RECEIVING A LIGHT BEAM FROM A SELECTED REGION OF AN ELECTRIC ARC AND TRANSMITTING IT WITHOUT ALTERATION INTO A SPECTROGRAPH, COMPRISES A SEALED ENCLOSURE HAVING A CONTROLLED ATMOSPHERE, A CONCAVE RECEIVING MIRROR WHICH IS ADJUSTABLE IN POSITION RELATIVE TO TWO PERPENDICULAR AXES, A FLAT MIRROR, A FLAT DIVIDING MIRROR WHICH DIVIDES THE BEAM INTO A MAIN FRACTION, WHICH FORMS A REAL IMAGE AT THE SPARK POINT OF THE SPECTROGRAPH, AND A SECONDARY FRACTION WHICH CONTROLS THE POSITION OF THE RECEIVING MIRROR.

June 13, 1972 J. M. VIRLOGET 3,669,546

DEVICE FOR SPECTROGRAPHIC- ANALYSIS OF A LIQUID METAL 5 Sheets-Sheet 1Filed Oct. 26, 1970 mm E @N & R a A. mm

June 13, 1972 J. M. VIRLOGET DEVICE FOR SPEOTROGRAPHIC ANALYSIS OF ALIQUID METAL 5 Sheets-Sheet 2 Filed Oct. 26, 1970 mm mm June 13, 1972 J.M. VIRLOGET 3,669,546

DEVICE FOR SPECTROGRAPHIC ANALYSIS OF A LIQUID METAL 3 Sheets-Sheet 5Filed Oct. 26, 1970 :v 2 mm mm vw United States Patent 3,669,546 DEVICEFOR SPECTROGRAPHIC ANALYSIS OF A LIQUID METAL Jean Marcel Virloget, LeMesnil-Saint-Denis, France, as-

signor to Socit Francaise dlnstruments de Controle et dAnalyses,Yvelines, France Filed Oct. 26, 1970, Ser. No. 83,801 Claims priority,application France, Oct. 28, 1969, 6936939 Int. Cl. G01 3/30, 3/00 US.Cl. 356-86 13 Claims ABSTRACT OF THE DISCLOSURE The invention isapplicable to the spectrographic analysis of any molten metal in afurnace.

It is an object of the present invention to provide a device whichenables spectrographic analysis of a liquid metal contained in a meltingor manufacturing furnace. More particularly, the device according to theinvention serves to collect and to transmit, without alteration, a lightbeam emitted in a furnace at the surface of a liquid metal to aspectrograph of known type.

It is also an object of the present invention to provide an assemblycomprising the above device and forming a functional combination of aspectrograph and a furnace particularly for metals or alloys at elevatedmelting temperatures.

The word furnace is used here in its widest sense to refer to allenclosures, particularly with refractory walls, adapted to contain aliquid metal, whether a known type of furnace, a convertor, a mixer orthe like. Likewise, the word metal services to indicate all materials,particularly metallic alloys, of which it is required to known at leastpartially the composition while they are in a liquid state under theinfluence of heat. It will be understood that the invention isparticularly applicable to iron metallurgy and to metallurgy in generalin which molten metals are produced in a temperature range of 300 to7800 C. The application of the invention is, however, not limited tothese possibilities.

It is known that it is extremely useful, particularly in electric steelfurnaces and with continuous flow installations, to be able to know inas short a time as possible the composition, or at least the content ofcertain elements, of a molten metal in a furnace. The production ofsteel has, with the use of oxygen, become a rapid process of the orderof twenty minutes in all, which in return requires rapid correctionalmeasures, and, consequently, analysis made as quickly as possible duringthe production. This requirement tends to render impractical, because itis too slow, the classical method, which consists in removing a sample,allowing it to solidify, polishing it roughly on one face and using itas an electrode at the spark point of a spectrograph, which isfrequently situated in a laboratory some distance from the furnace. In anumber of countries attempts have already been made to collect a lightbeam emitted by an electric are created at the surface of a fusion bathand to analyse this beam by means of a spectrograph. However, until nowthe various solutions proposed have not enabled a correct and constantanalysis which is industrially utilisable to be obtained. It is evident,in fact, that anlysis graphs should be obtained and reproduced at willby arcing on the metal liquid which possess a constant correlation withthe graph of the true composition of hte metal such as would beprovided, for example, by chemical analysis.

The difficulties to be overcome, which are numerous, include:

The elevated temperatures (1600 C. for steels) which prevail at theinterior of the furnace and the high sensitivity of spectrographs to theambient temperature (they are usually maintained at a stabilisedtemperature of A 0.);

The poor quality, from the optical point of View, of the atmosphere inthe vicinity of the molten bath (fumes metallic vapours-varying presenceof C0, C0 S0 0 etc.) and the high sensitivity of the light beam tovariations in this atmosphere. For steels, it should be possible tocollect and transmit without alteration, to the analysis spectrographrays the wavelength of which are between 1650 and 2800 A., metalloidsemitting rays mainly between 1650 and 2000 A. and metals mainly between2200 and 28 A. In certain cases, it should be possible to detectluminoous rays from certain bodies up to 3500 A. and even up to 8000 A.(7765 A. for the detection of potassium).

To give a single example, oxygen totally absorbs rays of wavelength lessthan 1860 A.

It is an object of the present invention to mitigate the difficultiesbriefly outlined above and also others which are discussed below.

An important object of the invention is to transmit without alterationto a spectrograph sufi'iciently spaced from a furnace a light beamprovided by an electric arc created at the surface of molten metal atthe interior of an industrial furnace.

A further object of the invention is to collect a light beam emitted bya selected region of an electric arc and to conserve to a satisfactoryextent the beam coming from this same region, in spite of the relativeinstability of the electric arc in space.

A still further object of the present invention is to provide a devicesuitable for transmitting a light beam including a widely differingrange of wavelengths, e.g. of 1600 to 8000 A. or more.

Another important object of the present invention is to provide acomplete industrially utilisable installation capable of providing in abrief time of the order of one minute a correct and reproducablespectrograph analvsis of a molten metal in a furnace.

According to the present invention a device for receiving andtransmitting a light beam emitted by a selected region of an electricare produced in a zone of elevated temperature, for example a furnacecontaining a molten metal, to an analysis apparatus such as aspectrograph having a spark point, an entry opening and an inlet slotfor a luminous beam, comprising a hollow and closed elongate enclosurehaving an orifice communicating with means for controlling theatmosphere in the interior of the enclosure, an inlet opening for anincident light beam disposed in one end wall and closed by a suitabletransparent material, a first outlet opening for a main fraction of thelight beam disposed in the opposite end wall and communicating with theinlet of the spectrograph by a sealed connecting element, an opticalsystem disposed within the enclosure and comprising at least a receivingmirror which receives the light beam from the inlet opening and reflectsit, the receiving mirror being mounted so as to be adjustable inposition with respect to at least one axis, a mirror which has anopening and which divides the light beam into a main fraction whichpasses to the spectrograph and a secondary fraction, a mechanism forcorrecting the position of the receiving mirror, this mechanism beingconnected to the receiving mirror, and a control elment connected to themechanism and having a beam detector disposed in the path of thesecondary fraction of the beam in such a manner that the position of thereceiving mirror is controlled in accordance with the quantity of lightreceived by the beam detector, the optical system producing a real imageof the selected region of the electric arc at the spark point of thespectrograph.

In one embodiment of the invention, the elongate enclosure surrounds thespark point of the spectrograph, the element connecting the first outletopening of this enclosure to the inlet of the spectrograph being formedby a hollow tubular sealing element containing, preferably, an opticalcondenser.

In another embodiment of the invention, the sealed connecting element isan optical fibre having one end disposed as the spark point and a secondend, preferably of rectangular section, adapted and fixed to the inletslot of the spectrograph.

According to another aspect of the invention, it is an object of theinventionto provide an assembly for spectrographic analysis of a metalinfusion comprising; a

. furnace having a refractory wall with a lateral opening,

a molten metal contained by the wall of the furnace at a level below thelateral opening, a generator producing an electric are between anelectrode and the surface of the molten metal, a hollow connectingelement secured at one end to the wall of the furnace around the lateralopening, a device as defined above secured to the second extremity ofthe said hollow connecting element around the inlet opening of thisdevice, and a spectrograph connected to the first outlet opening of thisdevice.

The invention will be more readily understood from the followingdescription of two embodiments thereof given by way of example withreference to the accompanying drawings, in which:

FIG. 1 is a general schematic partial view in section of an assemblyembodying the invention;

FIG. 2 shows a partial schematic view supplementing FIG. 1;

FIG. 3 is a partial view in section of a further assembly according to asecond embodiment of the invention.

'Before beginning the description, it is noted that the assembliesillustrated, which embody the invention, may be provided with numerousmodifications, which can be made by those skilled in the art, requiredfor adaption to the operation of the type of furnace and the nature ofthe liquid metal. 'It is accordingly to be understood that theembodiments given here are in no way limiting.

'Referring to FIG. 1, there is shown a furnace 1 having a thickrefractory wall 2 in which there is formed a lateral opening 3. Thisfurnace contains a liquid alloy 4 which is maintained molten by means ofheating, which is not illustrated. A spark generator 5 is mounted closeto the furnace 1 and is connected to an electrode 6 which extends intothe liquid bath 6 and to an electrode 7 which is maintained at a smalldistance above the surface of the liquid metal facing and in thevicinity of the opening 3.

Means other than the generator 5 illustrated could be employed providedthat they create an electric are 8 at the surface of the liquid metal 4,which performs the role of an electrode.

The are 8 is relatively unstable in the sense that it varies in shapeand position, the surface of the bath 4 not being not perfectly stilland causing displacement of the arc. Only the central region of the arcis relevant.

Around the opening 3 on the wall 2 of the furnace 1 there is secured oneend of a flexible hollow connecting element which is expansible, such asa light bellows 9, which can support large variation of temperaturewithout its length varying and which can absorb deformations in 4 thelateral and longitudinal directions. The other end of the bellows 9 isfixed to the front face of the front wall 11 of a device indicatedgenerally by reference numeral 10, which will be described in greaterdetail below.

The device 10 has, opposite the front wall 11, a rear wall 12 to whichis secured one end of a sealed connecting element such as a flanged tube13, the other end of which is secured to a spectrograph 14 of knowntype.

The device 10 comprises an elongate enclosure which is sealed and whichhas a connecting orifice 16 by means of which it can be connected to ameans (not shown) for allowing the atmosphere prevailing in the interiorof the enclosure 15 to be controlled. A vacuum may be produced in theinterior of the enclosure 15, and then at a predetermined pressure a gasmay be emitted which is transparent to certain wavelengths (nitrogen,argon, helium etc.).

The bellows 9 is secured to the wall 11 around an inlet opening 17 whichis closed by a suitable material having the required transparency, suchas quartz disc 18.

In addition, within the thickness of the wall 11 thereis provided partof a refrigerating circuit 11a which is connected to a fluid circulationsystem and a passage 11b which opens outside the enclosure 15 through acircular orifice close to the quartz disc 18 and which communicates withthe interior of the bellows 9. This passage 11b is connected to a sourceof gas under pressure; in this way, a permanent flow of gas from theinlet opening 17 through the bellows 9 towards the furnace 1 can beestablished. The tube 13 is fixed to the wall 12 around a first outletopening 19, which may be formed either by a quartz disc analogous to thequartz disc 18 or, as in the present embodiment, by an optical condenser20 mounted in the tube 13.

The enclosure 15 has a second, lateral, outlet opening 21 closed by aquartz disc 22,

The enclosure 15 also contains an optical system which comprises:

A mirror 23, referred to as the receiving mirror, which in this exampleis a concave mirror of magnification 1 and which is disposed so as toreceive a light beam from the selected region of the are 8;

A fiat mirror 24 disposed to receive the beam reflected by the receivingmirror 23;

A flat mirror 25 having a central opening 26 disposed in the path of thebeam reflected by the mirror 24 and referred to as the dividing mirror.

Reference is now made to FIG. 2, which supplements FIG. 1 and in whichthe enclosure 15 has been omitted.

The receiving mirror 23 is mounted so as to be adjustable in positionwith respect to two perpendicular axes. In this example, it is supportedby a flexible spindle 27 at one point of its periphery; a rigid spindle28 fixed to the opposite point connects it to a permanent magnet 29which is mounted so as to be displaceable in opposite directions in theinterior of a winding 30 supported in the interior of the enclosure 15.This forms a mechanism for correcting the position of the receivingmirror 23. An identical assembly (not shown) disposed in another planeand connected to the mirror 23 at another point allows correction of theposition of the mirror around another axis. The mirror may be mounted ona ball and socket joint instead of being supported by the flexiblespindle 27.

Facing the mirror 25 there is disposed a flat mirror 31 which isassociated with a lens 32 and with a control member indicated generallyby reference numeral 33. The control member 33 comprises a beam detectorand is connected to the mechanism for correcting the position of thereceiving mirror 23.

In this embodiment, there are two groups of two photoelectric cells 34,35 disposed in opposite pairs in two perpendicular directions (a singlegroup is shown in FIG. 2). The photo-electric cells 34 and 35 of onegroup are connected to a differential amplifier 36 the output of whichis connected to a power amplifier 37. The output of the power amplifier37 is in turn connected to the correcting mechanism, more particularlyto the winding 30.

The above-described asembly shown in FIG. 2 can be placed in theinterior of the enclosure 15. In the case of FIG. 1, it is placed on theexterior, the mirror 31 being, as can be seen, disposed facing thesecond outlet opening 21 of the enclosure 15.

Returning again to FIG. 1, attention is again directed to thespectrograph 14. The spectrograph 14 will not be described in detailbecause it is of a well known type, but it is noted that thespectrograph 14 has a spark point 38 where an electric spark is producedwhen a sample of solidified metal is to be analysed, an entry opening 39and an entry slot 40.

The expression spark point is used here since it relates to theparticular case of an electric spark. It should however be understoodthat in general this expression refers to the region where the atoms ofthe material to be analysed are excited by means of an electric are orotherwise.

As shown in FIG. 1, the spark point 38 is located in the interior of theenclosure 15 and the closure of the second outlet opening 19 is ensured,with respect to sealing, by the condenser 20. This arrangement is notessential. The spark point could be on the exterior of the enclosure 15,for example outside the second outlet opening 19. In this case, the tube13 should be more elongated. In addition, a quartz disc can also beplaced in the outlet opening 19. The arrangement illustrated ispreferable because it avoids the use of this supplementary quartz discand thus it enables the spark point 38 to be located in the controlledatmosphere of the enclosure 15.

The operation of the arrangement described above will now be explained.

An electric arc 8 having been created at the surface of the moltenmetal, the device 10 is disposed so that the receiving mirror 23receives through the inlet opening 17 a beam enamating from the regionof the are which is of most interest. For convenience of explanation, itwill be assumed that it is a beam of visible light although it could becomposed of certain wavelengths outside the visible spectrum.

The mirror 23, by way of the mirror 24, reflects the beam onto themirror 25 which, in turn, divides the beam which it receives into a mainfraction 41 and a secondary fraction 42. The secondary fraction 42passes through the second outlet opening 21 and is reflected by themirror 31 and, by means of the lens 32 (which is preferably of variablefocal length) a real image of the arc 8 is formed at the centre of thefour photo-electric cells 34, 35. If the are 8 is displaced, its imageformed by the lens 32 is also displaced; the relative illumination ofthe photoelectric cells is modified and the differential amplifier 36transmits a correcting signal. This signal is converted into amodification of the intensity of the current passing through the winding30. The position of the magnet .29 changes and so does that of thereceiving mirror 23 in such a manner that the latter always receives alight beam coming from the same selected region of the are 8.

The main fraction 41 coming from the concave mirror 23 forms anotherreal image of the are 8 at the spark point 38. It will be noted that thecentral fraction of the beam received by the receiving mirror 23 isemployed for analysis by the spectrograph while the annular fractionwhich is less homogenous and more variable, is employed for correctingthe position of this mirror.

It is important that the beam should undergo the fewest possiblerefractions and reflections between the are 8 and the spectrograph 14.The invention is notable for the simplicity of the means used to receivethe beam issuing from the same region of the arc 8 and transmitting itwith the least alteration possible.

Other mirrors could of course be employed for elongating the path of thebeam and thus further spacing the spectrograph from the furnace, forexample, by forming successive intermediate images before the beamreaches the spectrograph. In any case, as has been mentioned, it isdesirable to limit the number. It will also be noted that the flatmirror 24 could be replaced by a dividing mirror having a centralreflecting part and an annular opening surrounding this reflecting part.The main fraction 41 would then be directed directly towards the sparkpoint 38, as in FIG. 1, and the secondary fraction 42 which passesthrough this new dividing mirror could be reflected from the enclosureby one or more other mirrors.

In practice, in many cases, as soon as the temperature of the moltenmetal to be analysed reaches a fairly elevated value, the device 10cannot be used by itself. It should be joined to the furnace 1 by meansof the bellows 9. Cold water is then supplied through the refrigeratingcircuit 11a and a gas under pressure is supplied to the passage 11b.This gas is selected in accordance with its transparency to thewavelengths employed, as is that in the enclosure 15. It passes throughthe bellows 9 and enters the furnace 1, into which it is discharged. Inthis way a controlled atmosphere is provided between the are 8 and theentry opening 17. At the same time, the metallic vapours which wouldhave a tendency to deposit on the quartz disc 18 are kept away. Thequartz disc 18 could be provided with a closure which would protect itwhile the arrangement is not in operation.

It is useful for the connecting element between the furnace and thearrangement 10 to be capable of supporting large difference oftemperature and also to be deformable sufliciently easily in its lateraland longitudinal directions. It is thus possible to effect a firstcentring of the receiving mirror 23 on the electric discharge arc byrelative displacement of the spectrograph and the arrangement 10, makinguse of the flexibility of the bellows 9. The angular amplitude of theadjustment of the mirror 23 is, in effect, limited.

Proceeding as described above, one encounters a second aspect of theinvention which comprises a complete assembly from the furnace to thespectrograph. This assembly can be modified in numerous different Waysdepending upon the furnace employed. "FIG. 3 shows one example. In thisfigure there is shown an installation in which parts which also appearin FIG. 1 have been indicated by the same reference numerals as in FIG.1.

FIG. 3 shows the constnuction of the Wall 11 with the refrigerationcircuit 11a and the passage 11b. Also, it shows that the exterior mirror31 is mounted with the required inclination at the base of a casing 43which is secured along the enclosure 15..

The main difference with respect to the embodiment of FIG. 1 is that,the furnace being of a different, smaller type, the bellows 9 is notsecured directly to the wall 2 of this furnace but to an intermediatesupport 44 which is cooled by an internal circulation of water. Theintermediate support 44 is adapted to receive, by means of screws andjoints, a nose-piece 45 which forms a con!- nection with the lateralopening 3 of furnace 1. To simplify the description, it is mentioned thesupport 4 is receivable in the external face of the wall of a furnace ofwhich the element 2 is the internal face, while the nose-piece 45 limitsthe lateral opening 3 formed in this wall.

A further embodiment of the invention, which is not illustrated in thedrawings because it is simple to understand, will also be mentionedbecause it can be used with advantage for light alloys, in particularwhen wavelengths of more than 2000 A. are to be transmitted.

According to this modification, the tube 13 ('FIG. 1) is omitted, and asa sealed connecting element an optical fibre is employed which is ofknown type formed of elementary filaments. One end of this fibre isdisposed at the spark point 38 and the other has an elongate rectangularsection corresponding to the inlet slot 40 of the spectrograph and issecured at this slot. The sealing of the enclosure 15 is preserved bygripping the optical fibre by means of a packing where it passes throughthe first outlet opening 19.

This embodiment of the invention provides a gain in light with respectto the embodiments of FIGS. 1 and 2.

By employing elementary fibres of about 15 1. diameter, they can bearranged in a single line and adjusted to the inlet slot 40. The limitof employment for wavelengths greater than 2.000 A. is due to the actualnature of the optical fibres; new materials could increase thepossibility of employing them. In any case, at present, with therestriction mentioned, one can employ fibres having several metreslength which enables the spectrograph -14 to be further spaced from thedevice 10 if necessary.

In the above description, for the purposes of explaining the apparatus,it has been stated that a lateral opening of the furnace is employed.The invention is however not limited to this arrangement. One could, asa modification, employ an upper opening in the furnace through which anoptical rod is pushed downwardly. This rod could be arranged to receivea beam emitted from the electric arc and to direct it upwardly and thento one side, when the situation would 'be as described above but at adifferent level. As in the case of the lateral opening in the furnace,this optical rod could be traversed by a flow of gas under pressure.

The results of an experiment carried out with an assembly such as thatshown in FIG. 3 will now be given. The quantity of molten metal wasabout three kilogrammes, and the spectrograph was of known type suitablefor wavelengths of 1700 to 3700 A.

The results obtained by analysis of the liquid metal are given on thetable below in comparison with the results of the analysis of the samemetal in its solid state obtained with the same spectrograph by thenormal sampling method.

The results are expressed in the form of relative error in both caseswith respect to the content measured by analytical chemistry.

1 Or analysis impossible.

It will be noted that only for carbon between 0.1 and 0.7% and forphosphorus is the error occurring with the assembly according to theinvention greater, in a sufficiently clear manner, than the errorobtained by spectrographic analysis of solid metal; but these errors and4%) are of an amount which is acceptable in practice.

It will also be noted that for a high carbon content (2 to 5%) thespectrographic analysis of solid metal is impossible because of thepreferential path followed by the spark across the carbon. This awkwardphenomenon is less likely to occur with molten metal so that the presentinvention, in these conditions, enables an analysis to be carried outwhich cannot be carried out by normal spectrographic analysis.

I claim:

1. A device for receiving and transmitting a light beam emitted from aselected region of an electric are produced in a zone of elevatedtemperature, for example a furnace containing a molten metal, to ananalysis apparatus such as a spectrograph having a spark point, an entryopening and an inlet slot for a luminous beam, comprising a hollow andclosed elongate enclosure having an orifice communicating with means forcontrolling the atmosphere in the interior of the enclosure, an inletopening for an incident light beam disposed in one end wall and closedby a suitable transparent material, a first out-let opening for a mainfraction of the light beam disposed in the opposite end wall andcommunicating with the inlet of the spectrograph by a sealed connectingelement, an optical system disposed within the enclosure and comprisingat least a receiving mirror which receives the light beam from the inletopening and reflects it, the receiving mirror being mounted so as to beadjustable in position with respect to at least one axis, a mirror whichhas an opening and which divides the light beam into a main fractionwhich passes to the spectrograph and a secondary fraction, a mechanismfor correcting the position of the receiving mirror, this mechanismbeing connected to the receiving mirror, and a control element connectedto the mechanism and having a beam detector disposed in the path of thesecondary fraction of the beam in such a manner that the position of thereceiving mirror is controlled in accordance with the quantity of lightreceived by the beam detector, the optical system producing a real imageof the selected region of the electric are at the spark point of thespectrograph.

2. A device as claimed in claim 1, wherein, the elongate enclosureencloses the spark point of the spectrograph, and the element connectingthe first outlet opening of the enclosure to the inlet of thespectrograph being formed of a hollow sealed tubular element containingan optical condenser.

3. A device as claimed in claim 1, wherein the sealed connecting elementcomprises an optical fibre extending between the elongate enclosure andthe inlet of the spectrograph.

4. A device as claimed in claim 3, wherein the optical fibre has one enddisposed at the sparking point and a second end of rectangular sectiondisposed at the inlet slot of the spectrograph and fixed to the saidslot.

5. A device as claimed in claim 1, wherein the elongate enclosure has asecond outlet opening closed by a suitable transparent material disposedin the path of the secondary fraction of the light beam, the controlmember of the beam detector being disposed on the exterior of theelongate enclosure and the correcting mechanism coupled with thereceiving member being disposed within the elongate enclosure.

6. A device as claimed in claim 5, wherein the control member of thebeam detector comprises, in the path of the secondary fraction of thebeam, a lens which produces an image of at least a part of the initialelectric are, at least two photo-electric cells disposed in oppositionin the plane of the image, and electronic circuit connected 'to thephoto-electric cells, the circuit emitting a signal to the correctionmechanism coupled to the receiving mirror for correcting the position ofthis mirror in accordance with the intensity of the beam received by oneor the other of the photo-electric cells.

7. A device as claimed in claim 6, wherein the correcting mechanismcoupled to the receiving mirror comprises a winding connected to theelectric circuit, a permanent magnet freely movable in the interior ofthe winding, a spindle connecting the permanent magnet to a first pointon the mirror, the mirror being supported at a point spaced from thefirst point by a flexible spindle.

8. A device as claimed in claim 6, wherein the receiving mirror ismounted so as to be adjustable in position in the interior of theenclosure in at least two orthogonal planes, while two groups of twophoto-electric cells are disposed in opposition in a perpendiculardirection, two correction mechanisms being connected respectively to thetwo groups of photo-electric cells.

9. A device as claimed in claim 5, wherein the optical system disposedin the enclosure comprises a receiving mirror inthe form of a concavemirror of magnification substantially equal to 1 receiving the incidentbeam, a first flat mirror receiving the beam reflected by the receivingmirror and reflecting it towards the first outlet opening and towardsthe spark point, a second flat mirror having a central opening andreceiving the beam reflected by the first fiat mirror and dividing itinto a main fraction which passes through the central opening and asecondary fraction which is reflected through the second outlet opening.

10. An assembly for the spectrographic analysis of a molten metalcomprising a furnace having a refractory wall with an opening, a moltenmetal contained by the wall of the furnace, a generator producing anelectric are between an electrode and the surface of the molten metal, ahollow connecting element secured at one end thereof to the wall of thefurnace around the opening, a device as claimed in claim 1 secured tothe second end of said hollow connecting element around the inletopening of this device, and a spectrograph connected to the first outletopening of the device.

11. An assembly as claimed in claim 10, wherein the wall of theenclosure of the device provides in the vicinity of the transparentmaterial enclosing the inlet opening at least one orifice which is opento the interior of the hollow connecting element, this orificecommunicating with a passage which is connected to a source of gas underpressure in such a manner that a permanent flow of gas exists from theperiphery of the inlet opening of the enclosure through the hollowconnecting element to the interior of the furnace.

12. An assembly as claimed in claim 10, wherein the wall of the closedelongate enclosure contains, at least around the inlet opening, aninternal cooling circuit for the circulation of fluid.

13. An assembly as claimed in claim 11, wherein the opening is disposedin the upper wall of the furnace and a hollow optical rod which issealed is introduced through this opening, the said rod being connectedto the inlet of the device and a stream of gas under pressure forflowing through this rod.

References Cited UNITED STATES PATENTS 3,090,278 5/1963 Saunderson356-80 RONALD L. WIBERT, Primary Examiner V. P. McGRAW, AssistantExaminer US. Cl. X.R.

